Random musings - usually on a technical (nerdy) subject - perhaps related to amateur radio and things found at http://ka7oei.com

Thursday, March 28, 2013

Getting "Atomic" (WWVB) clocks to work indoors and in weak signal areas

Figure 1:
Shielded loop for WWVB (60 kHz). This goes
outside, away from interference sources.
This system has been in service for well
over a decade now and is
holding up nicely! Click on the image for a larger version.

Comment:

Due to a Blogspot bug, the original post of this article became "invisible" to readers after a minor edit and I could not make it reappear, so I had to re-post it as a "new" article - but it kept the original stats... go figure!

The problem:

About a dozen years ago a friend of mine came to me with a problem.

He works in a multi-story office building in downtown Salt Lake and in the main conference room there were two, large self-setting digital clocks.

Except that they didn't!

These radio-controlled clocks use a time signal broadcast from radio station WWVB near Fort Collins Colorado (some 360 miles - about 600 km) away on a longwave frequency of 60 kHz which means that even at that distance there's quite a strong signal day or night. The problem is that these same frequencies are the realm of devices that produce a lot of interference, namely fluorescent lights and electronic devices with switching power supplies - and the fact that this was a modern steel and concrete building amongst other steel and concrete buildings didn't help, either!

The solution?

Install an antenna outside, on the roof, where one had a reasonable chance to get a good signal with minimal interference and then pipe it into the building and couple it to the clocks!

This approach should work for anyone who lives in an area where their "local" longwave time signal (from WWVB, DCF77, MSF, JJY, etc.) is too weak to override the noise and attenuation experienced by clocks inside a building.

A shielded loop:

Since he was handy with metal work and welding, I sketched out a shielded loop and he returned a few days later with the setup seen in Figure 1, a piece of 1/2" galvanized electrical conduit that was bent into a straight-sided loop with each side being about 18" (approx. 45cm). While a circular loop would have been preferable, the picture loop was fairly easy to make with a standard conduit bender - a device that typically makes bends with only one choice of radius. When bending the conduit a bit of care was taken so that at the top, the two ends of the conduit that made the gap lined up with each other pretty well.

At the bottom of the loop was welded a steel plate that, when bolted to the aluminum box of the sort used to mount outdoor electrical outlets, held the loop vertical and onto this box was bolted a pair of clamps to allow it to be attached to a vertical pipe with all of the steel parts being cleaned and painted to prevent corrosion. Through the metal plate welded to the loop and into the conduit itself was drilled a hole of about 3/8" (approx. 10mm) diameter that allowed wires contained within the loop to be fished out and this hole was lined up with one drilled into the aluminum box. When RTV ("silicone") sealant was used around the weld and between this steel plate and the backside of the aluminum box, this provided a watertight seal and a means of bringing into the box the wires within the loop.

Note: If you weld to galvanized pipe as was done here be aware that toxic gasses may be produced and that proper ventilation is required!

Figure 2:
Inside the outdoor box - a signal amplifier. This boosts the weak signal
before it is conveyed on standard RG-6 TV coax. Toward the lower left
can be seen the wires emerging from the hole in the side of the box,
from the loop itself. Click on the image for a larger version.

Not obvious in Figure 1 is the fact that there's a gap in the loop at the top because this is an "H-Field" loop - that is, it's an antenna that ideally responds only to the magnetic component of the radio wave rather than the E (electric) field: This is important since it is more likely that nearby interference sources such as power lineswill more-strongly emit signals primarily in the E-field. This sort of loop antenna also has the advantage is that it has two deep nulls that can be steered to help reduce interferences from noise sources - provided, of course, that they don't happen to be in exactly the same direction as the desired signal!

Fishing the wire inside the loop was a bit tricky, but not too difficult - particularly if you use multi-conductor wire: Two pairs of telephone "cross-connect" wire were pushed into the hole at the bottom of the loop in each direction and at the top, they were then stripped, soldered and insulated to make a loop with a total of 4 turns. If you wish, you can use individual pieces of wire or, possibly, use some CAT-5 or telephone wire with multiple conductors to achieve the total of 4 turns, but it is recommended that you do not use shielded cable - since you already have a shield, anyway!

After this a piece of scrap plastic pipe or garden hose was cut to maintain the gap at the top of the loop to prevent the ends from touching and "shorting out" the magnetic field of the received signal. This was then insulated with high-quality electrical tape, reinforced with some tubing, and then taped again.

Comment: It is possible to make a multi-turn, shielded loop from coaxial cable supported by a nonmetallic frame of plastic or wood. Contact me if you want to know how this is done.

Outdoor amplifier:

Inside the metal box to which the loop is attached is a simple 2-stage signal amplifier and this boosts the received signal by about 30dB (1000-fold) prior to its being sent down into the building via standard RG-6 type TV coaxial cable. RG-6 and F-type connectors are chosen here since these are readily available worldwide and cheap, but practically any coaxial cable would work since neither loss or impedance are critical here! All that matters is that it will survive outdoors, exposed to the sun, and that you can make reliable connections to it.

(Note: If you have a bunch of old RG-59 coax with F-connectors on it for which you are tying to find a use, this would work fine for that!)

Figure 3:
A diagram showing the outdoor amplifier and loop along with two versions of the indoor amplifier.
Version 1 uses a high-power high-speed op amp while Version 2 uses the more readily-available
LM386 audio amplifier. Please note the warning about different versions of the LM386 andthe maximum allowed power supply voltages!Click on the image for a larger version.

If you look at the schematic diagram you'll see that the shielded loop (L101) has in parallel with it a capacitor (C101) that is used to broadly resonate it at the 60 kHz frequency of WWVB. While not strictly necessary in this area (Utah, U.S.A) with its strong signal from WWVB, it does help make the loop antenna a bit more efficient and could be of benefit in areas where the signal is weak because of distance and/or propagation - but remember that this would NOT help if the signal is being submerged in a local interference/noise source: You must carefully locate the antenna and rotate it to minimize that problem!

The value of the capacitors used to resonate this loop depend on the specific construction of your loop and if you wish to do it, you'll need a signal generator that goes at least as high as the frequency of interest (in my case, I used an audio generator since it was handy!) and an oscilloscope. I simply connected a 100 ohm resistor across the loop and then used a 1k series resistor to feed it from the signal generator and then connected an oscilloscope directly across the loop, the signal generator and the 'scope sharing a common ground. The frequency of the signal generator was then varied from around 20 to 200 kHz and a definite (but very broad) peak was noted: If the frequency of the peak is too high in frequency, add more capacitance and if it's too low, reduce the capacitance.

It just so-happened that with the 4-turn loop of the dimensions noted, four 0.1uF plastic capacitors in parallel gave a resonant peak "near-ish" 60 kHz and if you are building a loop of roughly those dimensions, 0.4uF would be a reasonable starting point. It's important to note, however, that you should not use ceramic capacitors (which are usually disk-shaped) to resonate the loop as these vary wildly with temperature: ONLY use plastic capacitors such as Mylar or Polycarbonate. Old PC power supplies often have use these sorts of capacitors on the filtering of their AC power inputs.

The use of 0.4uF to resonate the loop at 60 kHz implies that the loop itself has inductance of around 17.6 uH. If larger wire than the #24 AWG is used, expect the inductance to be slightly lower as a result. While the diameter of the loop itself isn't particularly critical, one can count on 4 turns on a larger-diameter loop having more inductance and needing less capacitance while the same number of turns on a smaller loop will need more capacitance. In general, the receive efficacy of a small (compared to the wavelength of the received signal) loop like this is related to the area of the the inside of the loop, so one should avoid making it too much smaller than this, although adding more turns to maintain the same or increase inductance and improve the efficiency will help - but don't go overboard and use too many turns as the parallel capacitance of such wire can, again, introduce losses.

Finally, one could, in theory, use some ferrite core antenna material such as that found in a discarded AM radio - and used in the radio-controlled clock itself, but to work properly a different amplifier circuit would be required, something that is beyond the scope of this article.

A few comments on parts:

The outdoor amplifier doesn't use any exotic or particularly hard-to-get parts so there is a bit of leeway on what will work. The transistors themselves are typical garden-variety small-signal silicon NPN transistors and practically any type similar to the 2N3904 (or better!) will work.

Aside from making sure that the loop itself is resonant at the receive frequency, the only other part that is somewhat critical is L102, the choke that passes the DC to power the amplifier, but blocks the RF. Since the impedance of the output amplifier is on the order of 50-100 ohms, one should pick the inductor so that its own impedance at the receive frequency should be at least 4 times (10 times is better!) this value to avoid the choke causing loading of the signal. For this, we should determine the inductive reactance, the formula for which is:

Since we want at least 400 ohms (that is, 4 times an assumed worst-case 100 ohms output impedance) of XL at 60 kHz, we find that we need at least 1 milliHenry of inductance. Since this is the lowest inductance that we should use we would actually prefer a choke that was several times higher than this.

When I built this circuit I happened to find a bifilar choke on the AC input of a discarded switching power supply and each half this choke measured as being 40 mH. Some of these chokes look like small transformers, but when they are used they are connected so that each half is placed in series with the AC power line and offers "common-mode" filtering of the power supply. Their inductance typically varies from a few hundred microHenries to 10's of milliHenries and since they are usually dual chokes, one can usually wire both halves in series, and since the inductance is related to the square of the number of turns, this will quadruple the inductance, making a choke with single-winding inductance as low as 300 microhenries potentially usable. One caution with wiring them in series: Observe the "polarity" of the winding because getting it wrong can cause the inductance to cancel out rather then increase!

On the indoor amplifier described below I didn't have another one of those chokes handy for L201 so I rummaged around and found one on the order of 4.7 mH - a value that we know from the calculations above to be entirely adequate.

Indoor amplifier and power inserter:

Figure 4:
The indoor amplifier, the component side of the board. This is the
"Version 1" circuit shown in Figure 3. L201, a small "solenoid"
choke can be seen in the lower-right corner of the board.Click on the image for a larger version.

Inside is located another box that contains yet another amplifier and a means of inserting power onto the RG-6 coaxial cable to power the outdoor amplifier.

The "Version 1 in Figure 3 is based on a high-power, fast op amp - the LM7171 - and this device is capable of easily driving several low-impedance loads such as the loops that were placed near the clocks.

Unfortunately, the LM7171 op amp is rather specialized, somewhat expensive and can be difficult to obtain (DigiKey in the U.S. sells it) and there are other high-speed, high-output op amps of the sort usable in video distribution amplifiers that could be substituted: I happened to use the LM7171 since I had some on hand! The circuit itself was quite simple and was built on a small piece of phenolic prototype board and placed in a small, plastic enclosure - both having been obtained from Radio Shack.

Perhaps a better choice for the average builder might be the venerable and more commonly-available LM386 audio amplifier denoted as "Version 2", also in Figure 3. This amplifier can operate up to 100 kHz with good output level and gain and is inexpensive and easy to use - Additional details on this circuit variation may be found later in this article.

As noted above, the value of L201 or L301 - as with L102 - the choke on the indoor unit, isn't critical, but it should be at least 1 millihenry for good results. A word of warning here: This choke should be large enough to be capable of withstanding a momentary short on the coaxial cable without damage! This is important since it's very easy to accidentally short out an F-type coaxial connector while attaching it. As it turns out, those very small molded chokes that look like 1/4-1/2 watt resistors will probably burn open (or melt and short out internally rendering them useless) if the full DC current of the power supply even momentarily flows through it!

You'll also note that across L201 (or L301) on the indoor unit is D201 (or D301), a 1 watt Zener diode - but the exact voltage isn't important and it could be anything from 5 to 15 volts. The purpose of this diode is to protect the amplifier (either the LM7171 or the LM386 if you use that) from the "zap" produced by L201/L301 if the coax output (to the antenna) is accidentally shorted out and then unshorted. If this happens, the magnetic field collapses in L201/L301 will collapse and the resulting high-voltage spike may destroy the amplifier chip! R201/R301 is also included to current-limit the voltage spike that results from the shorting of the coax (and the subsequent inductive spike) and should be included as well!

Installing the outdoor antenna:

When installing the antenna it should be oriented that the plane of the loop is pointed toward the transmitter. What this means is that if you were to stick your arm through the center of the loop it would be pointing toward the nulls of the antenna (e.g. 90 degree off from the transmitter) so if you have an interfering source, careful rotation of these nulls can be used to reject it. Note also that this antenna is bidirectional in that there are two peaks (and two nulls), each set of peaks (and nulls) being 180 degrees apart from each other.

One of the lesser-known corollaries of Murphy's law states that when installing an antenna, it will be discovered that the desired signal and the interference always come from the same direction, so you may have to relocate the loop to change the orientation of the two if you discover this to be true in your case! Fortunately, noise sources at these low frequencies cannot radiate effectively over large distances so moving the antenna a few meters/feet from the possible source may provide sufficient isolation to obtain a usable signal.

If you can locate the antenna on the roof there are usually relatively few sources of intereference, but one of these can be wireless data links. These are often mounted on masts and these days, typically feature "Power over Ethernet" (POE) and they often contain a number of internal switching supplies. If you have one of these, resist the temptation to mount the receive antenna within several feet/meters of the unit - or on the same mast!

It is also recommended that one avoid mounting the receive loop within 10 feet (3 meters) of a large metal structure such as the side of a building or a large air-handling unit as these can affect the signal level. Finally, although it is not particularly sensitive to height, it is best if one can locate the antenna at least 3 feet (about a meter) above the roof itself - particularly if it is metal.Indoor coupling loops:

Figure 5:
The bottom side of the indoor amp ("Version 1" in
figure 3) showing the F-connectors to
which the RG-6 coax that goes
to the coils that are placed near the receivers.Click on the image for a larger version.

Because the low-frequency signal at 60 kHz has such a long wavelength, it simply won't go very far at the signal levels being output by the amplifier so a coupling loop needs to be placed within a few feet (a meter or so) of each clock at most. If there are several clocks - or even dozens of clocks - one must assure that the signal is properly distributed and that there is a loop within "range" of any clock needing a signal.

Practically speaking, a multi-output video distribution amplifier could be used with each output driving its own loop, but one should be aware that some inexpensive units may tend to oscillate (and therefore not work properly - possibly causing interference on other frequencies) if they aren't presented with a resistive load!

For the coupling loops I used a scrap piece of 4-conductor telephone wire in a loop about 18 inches (45cm) diameter and wired such that it formed a 4-turn loop, the end of which was connected to an F-type coaxial fitting. As it happened, I also used capacitors to resonate the coupling loops as well using the same method as above using an oscilloscope and signal generator, but since I used a different dimension and number of turns of wire for the loop, the capacitor values were different. Practically speaking I probably didn't really need to resonate the loops since they radiated extremely well. Since the two clocks were on opposite sides of the room, a separate run of RG-6 coax was used to connect the coupling loop near each clock to the indoor unit. Note that TV-type splitters will not work at this low frequency!

The coupling loop should be placed within several feet (a meter or so)
of the clock that needs the signal. In the case of the installation in
the office building, the loops were simply laid atop the ceiling tile
directly above the clock - although it would have been possible to have
dropped them into the wall space behind the clock. Depending on the
quality of the signal being intercepted by the rooftop amplifier and the
amount of interfering signal within the building, it may be necessary
to experiment to determine the optimal positioning of the coupling loop
to the receiver.

One point of possible concern is that typical RG-6 uses aluminum shields which make it difficult to solder for the indoor loops so I used some chassis-mount F-connectors to which the loop's conductors were soldered. While the connector is preferred - especially since it allows the loop to be removed - one could probably get away with stripping bare an inch (2cm) of RG-6 aluminum shield, putting a bit of light grease on it (preferably some anti-oxidant grease used for electrical wiring, although petroleum jelly will work) and tightly wrapping some bare copper wire on the shield to which the end of the loop may be connected: The grease is recommended to minimize oxidation of the aluminum-copper connection. Once this is mechanically secure, carefully apply heat-shrink tubing (but not with so much heat as to melt the coaxial cable's foam dielectric) or secure it with a tight wrapping of electrical tape.

Choosing a power supply:

An important point about the power supply is that it SHOULD NOT be a switching supply as that type of power supply will likely generate an interfering signal that is very close to the frequency that you are trying to receive! In some countries, these older types of adapters are no longer available new, but they should still be plentiful as older, surplus items - and you probably have a suitable, orphaned unit laying around your house! If the power supply you choose has some "heft" to it there's probably an old-fashioned iron transformer in side, but if it seems as though it's an empty box, it's probably a "switcher" and should not be used!

While a regulated 12 volt "wall wart" (power adapter) would have been
preferred, a cheap, unregulated "12 volt" DC unit (about 15 volts unloaded) was used with good
results.

Use the correct version of the LM386 if you use the "Version 2" indoor amplifier:

At this point I will remind the would-be constructor that if other than the "-4" version of the LM386 is used, the power supply voltage applied to the LM386 must not exceed 12 volts. If you find that the voltage is, say, 13-14 volts, the easiest thing to do is to put in series with the LM386 (before the point on the diagram marked "V+") several silicon diodes such as those in the 1N4001 series. Each diode will drop the voltage by about 0.55 volts (e.g. 4 diodes in series will drop a bit more than 2 volts) and simply insert enough of them in series to drop the voltage to a safe level. A series resistor to could also be used to drop the voltage, but one would have to determine the correct value.Adjustments and cautions:

The main caveat of a system like this is that it is necessary to keep a reasonable distance between the outside antenna and a coupling loop. If there is at least one floor between the loop and the outside antenna and/or 10 meters of separation between the two, there should be no problem, but if they are anywhere in close proximity it is possible that the indoor and outdoor loops will couple into each other and cause the system to oscillate! While this oscillation is won't likely cause interference that extends more than a few 10's of meters from the coupling loops, it would prevent the system from working properly.

Other than the orienting of the receive loop and coupling loops as necessary, the only other adjustment is the gain control on the indoor amplifier (R203 for "Version 1" or R302 for "Version 2)). In the case of R203, it's likely that mid-rotation will provide sufficient signal from the coupling loops while on the LM386 version, a starting point would be 3/4 of the maximum.

Figure 6:
The indoor box showing front-panel
connectors. One goes to the roof-mounted
antenna to supply its power and to receive
the 60 kHz signal from WWVB while the
other connectors go to the loops that are
placed near the indoor receivers.Click on the image for a larger version.

Note that unless you live within 500 miles (800km) of the transmitter, it's possible that the signals will only be good enough for the clock to set itself during the night when the signals are likely stronger due to propagation, but it's also likely that many of the interfering sources (such as building lighting, computer monitors, etc.) may be powered down.Performance over the years:

This system (using the "Version 1" amplifier based on the LM7171) was originally installed in about 1998-1999 and has been in nearly continuous use ever since. In that time, it has failed twice:

The first time it failed was a few months after installation and the cover was removed from the outdoor box to inspect the circuit, but after allowing the accumulated water to pour out and the circuit board to dry, it started working again! Additional sealing of the outdoor box with RTV and the drilling of two small (1/16", 1-2mm) drain holes in the bottom prevented this from happening again!

The second time it failed was in 2012 and it was discovered that the filter capacitors in the plug-in wall transformer had failed and that there was too much AC ripple on the DC supply. To fix this, the wall transformer was replaced and C206 (a 1000 uF) capacitor was added to the indoor unit to provide redundant filtering.

The pictures on this page were taken recently after the unit had been in service for well over a decade, but was being checked out after the most recent repair.

Use with other time signals:

In the U.S., the signal that is used to automatically set these clocks is transmitted by WWVB on 60 kHz from Fort Collins, Colorado. In other parts of the world there are other, similar, time stations such as MSF in Great Britain, JJY in Japan, also on 60 kHz

Figure 7:
The Outdoor loop on the roof of the building where
it has been operating since the late 1990s. Because
WWVB is almost due east of this location, the loop's
plane is oriented in that same direction. It is a bit
closer to the roof than desirable, but it works fine
with the strong, local signal present.Click on the image for a larger version.

In Europe, there's also DCF77 on 77.5 kHz and all that would be necessary for this circuit to work on that frequency would be to resonate the loops on that frequency instead of at 60 kHz. In Japan, there's another JJY signal on 40 kHz and that, too, could be used if the loops are resonated appropriately - but one would, of course, choose the 40 or 60 kHz depending on which one was strongest in your area. Finally, there is also BPC in China on 68.5 kHz.

Comment:In addition to the frequency differences, these different stations broadcast different time codes. Unless your clock is specifically designed to work with multiple types of time signals, it will probably work only with the broadcast that is typically received in the country in which it was originally sold.

The difference between these "Radio Controlled" clocks and GPS-based clocks:

It's important to note that these radio-controlled clocks are not the same as GPS-based clocks as the two systems operate at very different radio frequencies: WWVB operates at a frequency of 60 kHz while consumer-grade GPS receivers operate at higher than 1.5 GHz.

A system such as this would not be appropriate for GPS clocks which rely on signals from orbiting satellites that are nearly 30000 times higher in frequency!

Is your "Atomic" clock no longer working?

The first thing to check is that the battery in your clock isn't nearly dead: The display on these clocks often continue to work when the battery has become too weak to operate the receiver used to get the time signals.

If you find that the clock still doesn't work after you have replaced the battery, it may be due to local interference that has recently popped up - perhaps you have a new electronic device such as a TV or computer - that is near the clock's location? Maybe one of these devices uses a power adapter that is radiating a signal? Remember that these adapters/devices are still on even if they seem to be powered off!

Is there a nearby electronic lamp (a compact-fluorescent or an LED) that is often left on in the wee hours of the morning when the clock is trying to receive the signal?

If you can't get the clock to synchronize even after replacing the battery and relocating a possible interference source the next step is to place it next to a window that faces 90 degrees away from the location of the transmitter - that is, if WWVB or DCF77 is to the east or west of you, place it in a north or south window. If you can safely place the clock outside, out of the weather and away from electrical sources, that would be good to try as well. If the clock synchronizes when placed outside, you will need to do a bit of sleuthing or, perhaps, find a different place for the clock.

If none of this works, it could be that the clock itself is broken and just won't synchronize - but it should still work as a "normal" clock.

Finally, it would seem that some models of clocks (particularly of the "SkyScan" brand) have stopped working after mid/late 2012, apparently due to a bug in their circuitry: Read about the problem in these posts:

I've occasionally been asked "Where can I get a system like this"? The answer is a bit tricky as any system involving potentially involving radio frequencies is subject to FCC rules. For personal use, one would take care to avoid doing something that could cause interference and considering the frequencies involved, one could reasonably be assured that a system like this would be generally incapable of doing so - even if it were to malfunction in some bizarre way!

Were a system like this to be actively marketed and sold then it would be incumbent on the manufacturer/seller to assure proper FCC (or your country's equivalent) compliance - a process that can be both expensive and arduous!